RF transceiver incorporating dual-use PLL frequency synthesizer

ABSTRACT

A dual-use PLL frequency synthesizer for use in a transceiver is capable of operating as a local oscillation generator in a receiving mode and as a transmitter in a transmitting mode. The PLL frequency synthesizer includes a digital processor, a Digital-to-Analog Converter (DAC), a low pass filter and a phase locked loop. The digital processor generates a digital signal, in which the digital signal is a modulated digital signal in the transmitting mode, and the digital signal is a reference digital signal in the receiving mode. The DAC converts the digital signal to an analog signal, and the low pass filter filters the analog signal to produce a filtered analog signal. The phase locked loop up-converts the filtered analog signal to an RF signal. In the transmitting mode, the RF signal is a modulated RF signal, and in the receiving mode, the RF signal is a reference RF signal.

BACKGROUND

1. Technical Field

The present invention relates to wireless communications and, moreparticularly, wideband wireless communication systems.

2. Related Art

Communication systems are known to support wireless and wire linedcommunications between wireless and/or wire lined communication devices.Such communication systems range from national and/or internationalcellular telephone systems to the Internet to point-to-point in-homewireless networks. Each type of communication system is constructed, andhence operates, in accordance with one or more communication standards.For instance, wireless communication systems may operate in accordancewith one or more standards, including, but not limited to, IEEE 802.11,Bluetooth, advanced mobile phone services (AMPS), digital AMPS, globalsystem for mobile communications (GSM), code division multiple access(CDMA), local multi-point distribution systems (LMDS),multi-channel-multi-point distribution systems (MMDS), and/or variationsthereof.

Depending on the type of wireless communication system, a wirelesscommunication device, such as a cellular telephone, two-way radio,personal digital assistant (PDA), personal computer (PC), laptopcomputer, home entertainment equipment, etc., communicates directly orindirectly with other wireless communication devices. For directcommunications (also known as point-to-point communications), theparticipating wireless communication devices tune their receivers andtransmitters to the same channel or channels (e.g., one of a pluralityof radio frequency (RF) carriers of the wireless communication system)and communicate over that channel(s). For indirect wirelesscommunications, each wireless communication device communicates directlywith an associated base station (e.g., for cellular services) and/or anassociated access point (e.g., for an in-home or in-building wirelessnetwork) via an assigned channel. To complete a communication connectionbetween the wireless communication devices, the associated base stationsand/or associated access points communicate with each other directly,via a system controller, via a public switch telephone network (PSTN),via the Internet, and/or via some other wide area network.

Each wireless communication device includes a built-in radio transceiver(i.e., receiver and transmitter) or is coupled to an associated radiotransceiver (e.g., a station for in-home and/or in-building wirelesscommunication networks, RF modem, etc.). As is known, the receiver iscoupled to the antenna and includes a low noise amplifier, one or moreintermediate frequency stages, a filtering stage, and a data recoverystage. The low noise amplifier receives an inbound RF signal via theantenna and amplifies it. The one or more intermediate frequency stagesmix the amplified RF signal with one or more local oscillations toconvert the amplified RF signal into a baseband signal or anintermediate frequency (IF) signal. As used herein, the term “low IF”refers to both baseband and intermediate frequency signals. A filteringstage filters the low IF signals to attenuate unwanted out of bandsignals to produce a filtered signal. The data recovery stagedemodulates the filtered signal to recover the raw data in accordancewith the particular wireless communication standard.

As is also known, the transmitter includes a data modulation stage, oneor more intermediate frequency stages, and a power amplifier stage. Thedata modulation stage converts raw data into baseband signals inaccordance with the particular wireless communication standard. The oneor more intermediate frequency stages mix the baseband signals with oneor more local oscillations to produce RF signals. The power amplifierstage amplifies the RF signals prior to transmission via an antenna. Incertain RF transceivers, the transmitter may be implemented as atranslational loop transmitter.

A translational loop transmitter may include a digital processor,digital to analog converter (DAC), low pass filter, and a phase lockedloop (PLL). The digital processor, in general, produces a digitalversion of the desired RF spectrum at some intermediate frequency (e.g.,26 MHz for GSM). The DAC converts the digital signals into the analogdomain, which are subsequently filtered by the low pass filter. Thetranslational loop translates the frequency of the analog signalsoutputted by the low pass filter to the desired radio frequencies.Specifically, a mixer in the feedback path of the translational loopuses an RF reference signal provided by a PLL frequency synthesizer toperform this “translation” of the IF signal to the desired RF frequency.

Thus, in a typical conventional RF transceiver architecture, thetransmitter is designed around a translational loop where a separate PLLfrequency synthesizer provides an RF frequency signal to thetranslational loop. In the receive mode, the PLL frequency synthesizerprovides the RF frequency to the receiver. However, translational loopsare expensive, consume large amounts of power and occupy a large amountof die space in RF transceivers. Therefore, what is needed is a lowpower and minimum cost RF transceiver architecture that operates using aconventional PLL frequency synthesizer in both a transmitting mode forgenerating the transmitted signals and a receiving mode for generatingthe local oscillations mixed with the received signals.

SUMMARY OF THE INVENTION

The present invention is directed to apparatus and methods of operationthat are further described in the following Brief Description of theDrawings, the Detailed Description of the Invention, and the claims.Other features and advantages of the present invention will becomeapparent from the following detailed description of the invention madewith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredwith the following drawings, in which:

FIG. 1 is a functional block diagram illustrating a communication systemthat includes a plurality of base stations or access points (APs), aplurality of wireless communication devices and a network hardwarecomponent;

FIG. 2 is a schematic block diagram illustrating a wirelesscommunication device as a host device and an associated radio;

FIG. 3 is a schematic block diagram illustrating an exemplary constantenvelope radio transceiver incorporating a dual-use PLL frequencysynthesizer, in accordance with embodiments of the present invention;

FIG. 4 is a schematic block diagram of an exemplary dual-use PLLfrequency synthesizer in accordance with embodiments of the presentinvention;

FIG. 5 is a schematic block diagram illustrating exemplary components ofa digital processor for use in the PLL frequency synthesizer of thepresent invention;

FIG. 6 is a schematic block diagram illustrating an exemplary digitalmodulator for use in the digital processor of the PLL frequencysynthesizer of the present invention;

FIG. 7 is a schematic block diagram illustrating an exemplary dual-notchre-quantizer for use in the digital processor of the PLL frequencysynthesizer of the present invention;

FIG. 8 is schematic block diagram illustrating an exemplary phase andfrequency detector (PFD) for use in embodiments of the presentinvention;

FIG. 9 is a circuit schematic illustrating an exemplary charge pump andloop filter combination for use in embodiments of the present invention;

FIG. 10 is a schematic block diagram illustrating an exemplary variableenvelope radio transceiver incorporating the dual-use PLL frequencysynthesizer of the present invention; and

FIG. 11 is a flowchart illustrating one method of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram illustrating a communication system10 that includes a plurality of base stations or access points (APs)12-16, a plurality of wireless communication devices 18-32 and a networkhardware component 34. The wireless communication devices 18-32 may belaptop computers 18 and 26, personal digital assistants 20 and 30,personal computers 24 and 32 and/or cellular telephones 22 and 28. Thedetails of the wireless communication devices will be described ingreater detail with reference to FIGS. 2-9.

The base stations or APs 12-16 are operably coupled to the networkhardware component 34 via local area network (LAN) connections 36, 38and 40. The network hardware component 34, which may be a router,switch, bridge, modem, system controller, etc., provides a wide areanetwork connection 42 for the communication system 10. Each of the basestations or access points 12-16 has an associated antenna or antennaarray to communicate with the wireless communication devices in itsarea. Typically, the wireless communication devices 18-32 register withthe particular base station or access points 12-16 to receive servicesfrom the communication system 10. For direct connections (i.e.,point-to-point communications), wireless communication devicescommunicate directly via an allocated channel.

Typically, base stations are used for cellular telephone systems andlike-type systems, while access points are used for in-home orin-building wireless networks. For example, access points are typicallyused in Bluetooth systems. Regardless of the particular type ofcommunication system, each wireless communication device and each of thebase stations or access points includes a built-in radio and/or iscoupled to a radio. The radio includes a transceiver (transmitter andreceiver) for modulating/demodulating information (data or speech) bitsinto a format that comports with the type of communication system.

FIG. 2 is a schematic block diagram illustrating a wirelesscommunication device 18-32 as a host device and an associated radio 60.For cellular telephone hosts, the radio 60 is a built-in component. Forpersonal digital assistants hosts, laptop hosts, and/or personalcomputer hosts, the radio 60 may be built-in or an externally coupledcomponent.

As illustrated, the host wireless communication device 18-32 includes aprocessing module 50, a memory 52, a radio interface 54, an inputinterface 58 and an output interface 56. The processing module 50 andmemory 52 execute the corresponding instructions that are typically doneby the host device. For example, for a cellular telephone host device,the processing module 50 performs the corresponding communicationfunctions in accordance with a particular cellular telephone standard.

The radio interface 54 allows data to be received from and sent to theradio 60. For data received from the radio 60 (e.g., inbound data), theradio interface 54 provides the data to the processing module 50 forfurther processing and/or routing to the output interface 56. The outputinterface 56 provides connectivity to an output device such as adisplay, monitor, speakers, etc., such that the received data may bedisplayed. The radio interface 54 also provides data from the processingmodule 50 to the radio 60. The processing module 50 may receive theoutbound data from an input device such as a keyboard, keypad,microphone, etc., via the input interface 58 or generate the dataitself. For data received via the input interface 58, the processingmodule 50 may perform a corresponding host function on the data and/orroute it to the radio 60 via the radio interface 54.

Radio 60 includes a host interface 62, a digital receiver processingmodule 64, an analog-to-digital converter 66, a filtering/gain module68, a down-conversion module 70, a low noise amplifier 72, receiverfilter module 71, a transmitter/receiver (Tx/RX) switch module 73, alocal oscillation module 74, a memory 75, a digital transmitterprocessing module 76, a digital-to-analog converter 78, a filtering/gainmodule 80, an IF mixing up-conversion module 82, a power amplifier 84, atransmitter filter module 85, and an antenna 86. The antenna 86 isshared by the transmit and receive paths as regulated by the Tx/Rxswitch module 73. The antenna implementation will depend on theparticular standard to which the wireless communication device iscompliant.

The digital receiver processing module 64 and the digital transmitterprocessing module 76, in combination with operational instructionsstored in memory 75, execute digital receiver functions and digitaltransmitter functions, respectively. The digital receiver functionsinclude, but are not limited to, demodulation, constellation demapping,decoding, and/or descrambling. The digital transmitter functionsinclude, but are not limited to, scrambling, encoding, constellationmapping, modulation. The digital receiver and transmitter processingmodules 64 and 76 may be implemented using a shared processing device,individual processing devices, or a plurality of processing devices.Such a processing device may be a microprocessor, micro-controller,digital signal processor, microcomputer, central processing unit, fieldprogrammable gate array, programmable logic device, state machine, logiccircuitry, analog circuitry, digital circuitry, and/or any device thatmanipulates signals (analog and/or digital) based on operationalinstructions. The memory 75 may be a single memory device or a pluralityof memory devices. Such a memory device may be a read-only memory,random access memory, volatile memory, non-volatile memory, staticmemory, dynamic memory, flash memory, and/or any device that storesdigital information. Note that when the digital receiver processingmodule 64 and/or the digital transmitter processing module 76 implementsone or more of its functions via a state machine, analog circuitry,digital circuitry, and/or logic circuitry, the memory storing thecorresponding operational instructions is embedded with the circuitrycomprising the state machine, analog circuitry, digital circuitry,and/or logic circuitry. The memory 75 stores, and the digital receiverprocessing module 64 and/or the digital transmitter processing module 76executes, operational instructions corresponding to at least some of thefunctions illustrated herein.

In operation, the radio 60 receives outbound data 94 from the hostwireless communication device 18-32 via the host interface 62. The hostinterface 62 routes the outbound data 94 to the digital transmitterprocessing module 76, which processes the outbound data 94 in accordancewith a particular wireless communication standard (e.g., IEEE 802.11a,IEEE 802.11b, Bluetooth, etc.) to produce digital transmission formatteddata 96. The digital transmission formatted data 96 will be a digitalbaseband signal or a digital low IF signal, where the low IF typicallywill be in the frequency range of one hundred kilohertz to a fewmegahertz.

The digital-to-analog converter 78 converts the digital transmissionformatted data 96 from the digital domain to the analog domain. Thefiltering/gain module 80 filters and/or adjusts the gain of the analogbaseband signal prior to providing it to the up-conversation module 82.The up-conversion module 82 directly converts the analog basebandsignal, or low IF signal, into an RF signal based on a transmitter localoscillation 83 provided by local oscillation module 74. The poweramplifier 84 amplifies the RF signal to produce an outbound RF signal98, which is filtered by the transmitter filter module 85. The antenna86 transmits the outbound RF signal 98 to a targeted device such as abase station, an access point and/or another wireless communicationdevice.

The radio 60 also receives an inbound RF signal 88 via the antenna 86,which was transmitted by a base station, an access point, or anotherwireless communication device. The antenna 86 provides the inbound RFsignal 88 to the receiver filter module 71 via the Tx/Rx switch module73, where the Rx filter module 71 bandpass filters the inbound RF signal88. The Rx filter module 71 provides the filtered RF signal to low noiseamplifier 72, which amplifies the inbound RF signal 88 to produce anamplified inbound RF signal. The low noise amplifier 72 provides theamplified inbound RF signal to the down-conversion module 70, whichdirectly converts the amplified inbound RF signal into an inbound low IFsignal or baseband signal based on a receiver local oscillation signal81 provided by local oscillation module 74. The down-conversion module70 provides the inbound low IF signal or baseband signal to thefiltering/gain module 68.

The analog-to-digital converter 66 converts the filtered inbound signalfrom the analog domain to the digital domain to produce digitalreception formatted data 90. The digital receiver processing module 64decodes, descrambles, demaps, and/or demodulates the digital receptionformatted data 90 to recapture inbound data 92 in accordance with theparticular wireless communication standard being implemented by radio60. The host interface 62 provides the recaptured inbound data 92 to thehost wireless communication device 18-32 via the radio interface 54.

As one of average skill in the art will appreciate, the wirelesscommunication device of FIG. 2 may be implemented using one or moreintegrated circuits. For example, the host device may be implemented ona first integrated circuit, while the digital receiver processing module64, the digital transmitter processing module 76 and memory 75 areimplemented on a second integrated circuit, and the remaining componentsof the radio 60, less the antenna 86, may be implemented on a thirdintegrated circuit. As an alternate example, the radio 60 may beimplemented on a single integrated circuit. As yet another example, theprocessing module 50 of the host device and the digital receiverprocessing module 64 and the digital transmitter processing module 76may be a common processing device implemented on a single integratedcircuit. Further, memory 52 and memory 75 may be implemented on a singleintegrated circuit and/or on the same integrated circuit as the commonprocessing modules of processing module 50, the digital receiverprocessing module 64, and the digital transmitter processing module 76.

The wireless communication device of FIG. 2 is one that may beimplemented to include either a direct conversion from RF to basebandand baseband to RF or for a conversion by way of a low intermediatefrequency. In either implementation, however, for an up-conversionmodule 82 and a down-conversion module 70, it is required to provideaccurate frequency conversion. For the down-conversion module 70 andup-conversion module 82 to accurately mix a signal, however, it isimportant that the local oscillation module 74 provide an accurate localoscillation signal for mixing with the baseband/IF or RF by theup-conversion module 82 and down-conversion module 70, respectively.Accordingly, the local oscillation module 74 includes circuitry foradjusting an output frequency of a local oscillation signal providedtherefrom.

In prior art radios 60 that use phase locked loop (PLL) frequencysynthesizers as the local oscillation module 74 to produce the localoscillations mixed with the transmitted and received signals, thetransmitter typically includes a translational loop that uses the RFfrequency of the PLL frequency synthesizer to “translate” the signalfrom an IF to the desired RF frequency, using a mixer. However, asdiscussed above, translational loops are expensive, consume largeamounts of power and occupy a large amount of die space in RFtransceivers.

Therefore, in accordance with embodiments of the present invention, theradio 60 can incorporate a dual-use PLL frequency synthesizer, therebyeliminating the translational loop of prior art transceivers, resultingin substantial die area savings. In one mode, when the radio 60 operatesas a receiver, the PLL frequency synthesizer provides the RF carrier forthe receiver. In the other mode, when the radio 60 operates as atransmitter, modulation is added to the PLL frequency synthesizeroutput, and hence the PLL frequency synthesizer operates as atransmitter.

FIG. 3 is a schematic block diagram of an exemplary RF transceiver 300incorporating a dual-use PLL frequency synthesizer 100, in accordancewith embodiments of the present invention. The RF transceiver 300includes a receiver 310 and a dual-use PLL frequency synthesizer 100,labeled “LO-Generator or Transmitter.” The dual-use PLL frequencysynthesizer 100 operates as either LO-generator for the receiver in areceiving mode or as a transmitter in a transmitting mode.

The receiver 310 includes the low noise amplifier (LNA) 72,down-conversion module 70, complex bandpass filter 68, a pair of highdynamic range delta-sigma (ΔΣ) analog-to-digital converters 66 a and 66b and the digital demodulator 64. In receiving mode, the LNA 72amplifies the inbound RF signal 88 to produce an amplified inbound RFsignal. The LNA 72 provides the amplified inbound RF signal to thedown-conversion module 70, which directly converts the amplified inboundRF signal into an inbound low IF signal or baseband signal based onreceiver local oscillation signals 81 provided by the dual-use PLLfrequency synthesizer 100. The down-conversion module 70 includes a pairof mixers 77 and 79 for down-converting the amplified inbound RF signalinto inbound low IF or baseband in-phase (I) and quadrature-phase (Q)signals, respectively, using cosine and sine reference RF signals 81provided by the dual-use PLL frequency synthesizer 100.

The mixers 77 and 79 provide the I and Q signals to the complex bandpassfilter 68, which filters the I and Q signals to provide filtered I and Qsignals. The filtered I and Q signals are converted from analog todigital via the high dynamic range delta-sigma (ΔΣ) analog-to-digitalconverters 66 a and 66 to produce respective digital signals. Thedigital demodulator demodulates the digital signals and extracts thedigital transmit data.

In transmitting mode, the dual-use PLL frequency synthesizer 100receives outbound transmit data 94 and modulates the outbound transmitdata 94 to produce a modulated RF signal 108. The power amplifier 84amplifies the modulated RF signal 108 to produce the outbound RF signal98. In one embodiment, the dual-use PLL frequency synthesizer 100 iscapable of operating in either the receiving mode or in the transmittingmode, but not in both modes simultaneously. Therefore, a control signal102 is provided to the PLL frequency synthesizer 100 to select betweenthe transmitting mode and the receiving mode. In addition, a controlsignal 104 is provided to the receiver 310 to activate the receiver 310in the receiving mode.

FIG. 4 is a schematic block diagram of an exemplary dual-use PLLfrequency synthesizer 100 in accordance with embodiments of the presentinvention. The dual-use PLL frequency synthesizer 100 includes a digitalprocessor 140, digital-to-analog converter (DAC) 160, low pass filter(LPF) 120, a phase locked loop (PLL) 122 and a power amplifier 150. Intransmitting mode, the digital baseband data 94 enters the digitalprocessor 140, which performs the necessary pulse shaping and modulationto some intermediate frequency (IF) carrier f_(IF). The resultingdigital signal 112 is a modulated digital signal that is converted to ananalog signal 114 by the DAC 160. The LPF 120 filters out undesireddigital images of the IF signal. The PLL 122 then translates thefiltered IF signal 116 to the desired RF frequency and the poweramplifier 150 delivers the modulated RF signal 108 to the antenna.

In receiving mode, the digital processor 140 generates the digitalsignal 112 as a digital reference signal at a reference frequencyappropriate for providing an accurate local oscillation signal formixing with the inbound RF signal in the receiver. The digital signal112 is converted to an analog signal 114 by the DAC 160, and the LPF 120filters out undesired digital images of the IF signal. The PLL 122 againtranslates the filtered IF signal 116 to the desired RF frequency anddelivers the reference RF signal (I and Q local oscillation signals) 81to the receiver.

The PLL 122 includes a phase and frequency detector (PFD) 124, a chargepump (CP) 126, a lowpass loop filter (LPF) 128, a voltage controlledoscillator (VCO) 130, and several divider blocks 132 and 136 in thefeedback path that each divide the incoming signal by some integer. Thefixed divide-by-2 block 132 allows the PLL frequency synthesizer 100 toeasily generate in-phase (I) and quadrature (Q) carrier signals in theGSM bands around 850 MHz and 900 Mhz by tuning the VCO 130 appropriatelyaround 3.6 GHz. In receiving mode, the I and Q carrier (RF) signals 81are input to the receiver as local oscillation signals, while intransmitting mode, the output of the divide-by-2 block 132 is input tothe power amplifier 150 to deliver the modulated RF signal 108 to theantenna. The multi-modulus divide-by-N divider 136 in the feedback pathenables a feedback signal 118 to “lock” to the filtered IF signal 116.

A qualitative description of the receiving mode operation of the PLLfrequency synthesizer 100 of the present invention is as follows. Thedigital processor 140 generates a digital reference signal 112 tunedaround 20-40 MHz (but no greater than half the clock frequency of 338MHz), which is converted to a reference analog signal 114 by DAC 160.The DAC 160 operates at a high sampling rate so as to ensure adequateattenuation of digital images via the lowpass filter 120 prior to beinginput to the PFD 124 of the PLL 122. The reference analog signal 114 isfiltered by LPF 120 to produce a filtered reference analog signal 116 tothe reference input of the PFD 124. The output of the PFD 124 is anerror signal (in phase and/or frequency) between the filtered referenceanalog signal 116 and a feedback signal 118. The charge pump 126responds to the (UP,DN) control signals of the PFD 124 by either“pumping” current into the loop filter 128 or moving current out of theloop filter 128 and “pumping” it into ground. The current pulses of theCP 126 are filtered by the loop filter 128 thereby generating a smoothoutput voltage referred to as the “control voltage”, v_(ctrl).

The oscillation frequency of the VCO 130 of the PLL 122 is determined bythe control voltage, V_(CTRL), supplied by the loop filter 128. The VCO130 oscillation is input to the fixed divide-by-two block 132 to producethe reference RF signal 81 for transmission to the down-conversionmodule of the receiver. The divider block 136 is coupled to receive theoutput of the divide-by-two block 132 and to divide the output by aconstant divide ratio to produce the analog feedback signal 118 input tothe PFD 124.

Fine frequency tuning is provided by finely tuning the reference signalvia the digital processor 140. The DC gain of the PLL frequencysynthesizer 100 is equal to N so a change of Δf in the referencefrequency results in a change of the product of N and Δf at the VCO 130output. Conversely, changing the divider N 136 by 1 changes thefrequency of the reference RF signal 81 by an amount equal to thefrequency of the filtered reference analog signal 116.

A qualitative description of the transmitting mode operation of the PLLfrequency synthesizer 100 of the present invention is as follows. Thedigital processor 140 receives the digital baseband data 94 and performsthe necessary pulse shaping and modulation to produce the modulateddigital signal (e.g., an intermediate frequency (IF) carrier f_(IF)),which is converted to a modulated analog signal 114 by DAC 160. Themodulated digital signal 114 is filtered by LPF 120 to produce afiltered modulated digital signal 116 to the reference input of the PFD124. The output of the PFD 124 is an error signal (in phase and/orfrequency) between the filtered modulated analog signal 116 and afeedback signal 118. The charge pump 126 responds to the (UP,DN) controlsignals of the PFD 124 by either “pumping” current into the loop filter128 or moving current out of the loop filter 128 and “pumping” it intoground. The current pulses of the CP 126 are filtered by the loop filter128 thereby generating a smooth output voltage referred to as the“control voltage”, v_(ctrl).

The oscillation frequency of the VCO 130 of the PLL 122 is determined bythe control voltage, v_(CTRL), supplied by the loop filter 128. The VCO130 oscillation is input to the fixed divide-by-two block 132 andreceived and amplified by the power amplifier 150 to produce themodulated RF signal 108 for transmission to the antenna. The dividerblock 136 is coupled to receive the output of the divide-by-two block132 and to divide the output by a constant divide ratio to produce theanalog feedback signal 118 input to the PFD 124.

The PLL frequency synthesizer 100 further includes a precise crystaloscillator (X-TAL) 110 and a clock generator 115. The crystal oscillatorprovides a clock frequency to the clock generator 115, which generates asquare wave clock signal and provides the clock signal to the digitalprocessor 140 and the DAC 160. For example, in one embodiment, the clockgenerator 115 provides a 338 MHz clock signal to the digital processor140 and the DAC 160.

FIG. 5 is a schematic block diagram illustrating exemplary components ofa digital processor 140 for use in the PLL frequency synthesizer of thepresent invention. The processor 140 shown in FIG. 5 including aregister programmable with a variable frequency value 142 thatdetermines the frequency of the reference signal (e.g., 20-40 MHz). Anadditional register is programmable with a constant tuning value 143 inorder to compensate for inaccuracies in the crystal reference frequencyprovided by X-TAL 110 (shown in FIG. 4). A digital modulator 141 isoperable to receive the digital baseband data 94 in the transmittingmode and perform the necessary pulse shaping and modulation to producemodulated digital data 202. The modulated digital data 202, referencefrequency value 142 and the tuning value 143 are input to a summing node144 that combines the modulated digital data, reference frequency value142 and the tuning value 143 to produce combined signal 204. Inreceiving mode, the digital modulator 141 does not produce the modulateddigital data 202, and therefore, the combined signal 204 includes onlythe reference frequency value 142 and any tuning value 143.

A phase accumulator 145 comprises a delay element 147 whose output isproduced to a feedback loop to integrate the combined signal 204 usingsummation node 146. Thus, phase accumulator 145 generates a phase ramp206 corresponding to the combined signal 204. A modulator 148 of thedigital processor 210 performs the frequency modulation of the phasesignal 206. For example, such modulation function may be implemented bythe COordinate Rotation DIgital Computer (CORDIC) algorithm. As is knownin the art, the CORDIC rotates a basis vector of the complex plane (1,0)by an amout equal to the output of the phase accumulator 145. Theresulting complex number is the output of the modulator 148. Thus,modulator 148 modulates the phase signal output from the phaseaccumulator 145 and produces corresponding I & Q vector digital data.

The I vector digital data 208 is produced to a dual-notch Delta Sigmamodulator 149 that re-quantizes the high-resolution digitally modulatedsignal to a coarse 8-bit signal in order to ensure that quantizationnoise is spectrally shaped in a favorable fashion. The output of thedual-notch re-quantizer is the digital signal 112.

FIG. 6 is a schematic block diagram illustrating an exemplary digitalmodulator for use in the digital processor of the PLL frequencysynthesizer of the present invention. The digital modulator 141 includesa symbol mapper 302 and a pulse shaping block 304. The symbol mapper 302maps incoming data bits 94 to the symbols {+1, −1{, and the pulseshaping block 304 performs the narrowband pulse shaping filtering. Forexample, in Global System for Mobile Communications (GSM) applications,the filter applied for pulse shaping is a Gaussian filter with BTproduct of 0.30.

FIG. 7 is a schematic block diagram illustrating an exemplary dual-notchre-quantizer for use in the digital processor of the PLL frequencysynthesizer of the present invention. The dual-notch re-quantizer 149operates with a 22-bit input signal 208 that is quantized to an 8-bitsignal 112 at the output node. Quantization is performed by a quantizer212 that takes the 8 most significant bits (MSBs) as the output 112. Afeedback loop including summation node 214, delay elements 216, 218 and220, summation node 222 and gain 224, as shown in FIG. 7, performs thespectral shaping of the quantization noise such that the quantizationnoise is favorable for the operation of the synthesizer.

FIG. 8 shows details of a typical implementation of the PFD 124. Tworeset-able flip-flops FF1 and FF2 are coupled with an AND gate in afeedback loop. Assuming that the signals IN1 and IN2 are appropriate fordriving digital circuitry, the operation of the PFD 124 is as follows.After reset, the outputs UP and DN are LOW, or 0. If IN1 goes HIGH, or1, then UP goes HIGH. When IN2 goes HIGH, DN goes HIGH momentarily,resulting in a positive edge at the AND gate output. This edge resetsthe two flip-flops FF1 and FF2 to the initial state (UP,DN)=(0,0). Thus,any phase difference between the two signals IN1 and IN2 results in thePFD 124 residing in the state (UP,DN)=(1,0) for a duration of timeproportional to the phase difference between IN1 and IN2. Similarly, anydifference in frequency between IN1 and IN2 results in the PFD 124residing in either the state (UP,DN)=(1,0) or the state (UP,DN)=(0,1),depending upon the sign of the frequency difference.

FIG. 9 shows details of a typical implementation of the charge pump 126and loop filter 128 combination. The charge pump 126 responds to the(UP,DN) control signals of the PFD by either “pumping” current into theloop filter 128 or moving current out of the loop filter 128 and“pumping” it into ground. The charge pump 126 includes two equallyweighted current sources CS1 and CS2, each with a nominal output currentI_(CP), in an arrangement with two switches S1 and S2 controlled by UPand DN. Thus, it follows that the CP 126 essentially functions as anasynchronously clocked digital-to-analog converter (DAC) whose nominaloutput y_(CP)(t) depends upon the digital inputs UP and DN such that${y_{CP}(t)} = \left\{ \begin{matrix}{I_{CP},} & {{{if}\quad\left\{ {{UP},{DN}} \right\}} = \left\{ {1,0} \right\}} \\{0,} & {{{if}\quad\left\{ {{UP},{DN}} \right\}} = \left\{ {1,1} \right\}} \\{0,} & {{{if}\quad\left\{ {{UP},{DN}} \right\}} = \left\{ {0,0} \right\}} \\{{- I_{CP}},} & {{{if}\quad\left\{ {{UP},{DN}} \right\}} = \left\{ {0,1} \right\}}\end{matrix} \right.$The current pulses of the CP 126 are filtered by the loop filter 128thereby generating a smooth output voltage referred to as the “controlvoltage”, v_(ctrl). The loop filter 128 typically consists of passivecomponents, e.g., resistors R2 and R3 and capacitors C1, C2 and C3. Theloop filter 128 shown in FIG. 5 is a third-order loop filter because itcontains three poles.

Referring again to FIG. 4, the oscillation frequency of the VCO 130 ofthe PLL 122 is determined by the control voltage, v_(CTRL), shown inFIG. 9. The sensitivity of the VCO 130 to changes in the control voltageis referred to as the VCO gain, denoted herein as K_(VCO). Thissensitivity is typically specified in MHz per Volt. For example, typicalnumbers for VCO gain are in the range 20 MHz/V to 50 MHz/V. In apractical setting, the VCO 130 typically undergoes “calibration” as partof the operating the PLL 122. This calibration determines the operatingpoint of the VCO 130 and allows the VCO 130 to function over a widerange of frequencies.

The transmitter architecture shown in FIG. 4 is an example of a constantenvelope communications scheme, such as that used in GSM systems.However, embodiments of the present invention are not limited toconstant envelope communications schemes. For example, the modulationscheme of Enhanced Data rates for GSM Evolution (EDGE) systems is avariable envelope communications scheme.

FIG. 10 is a schematic block diagram illustrating an exemplary variableenvelope radio transceiver incorporating the dual-use PLL frequencysynthesizer of the present invention. This type of transmitter is alsoreferred to as a polar transmitter. As can be seen from FIG. 10, thedigital processor 140 splits the digital signal into an envelopecomponent 112 a and a phase component 112 b. The envelope signal 112 ais converted to an analog envelope signal 114 a by a first DAC 160 a,and the phase signal 112 b is converted to an analog phase signal 112 bby a second DAC 160 b. The analog envelope signal is filtered by a firstLPF 120 a to produce a filtered analog envelope signal 116 a, and theanalog phase signal is filtered by a second LPF 120 b to produce afiltered analog phase signal.

The PLL 122 converts the filtered analog phase signal 116 b to the RF,and combines the RF signal with the filtered analog envelope signal atamplitude modulated PA's 150 and 155. Each PA 150 and 155 generates arespective signal by effectively multiplying the filtered analogenvelope signal 116 b with the translational loop output. In GSM(constant envelope) mode, the RF output carrier is modulated in thesecond PA 150 by the filtered analog envelope signal 116 b, which inthis case, is set to one. As such, the output of the second PA 150 whilein GSM mode is in effect an unmodulated RF carrier. A 2×1 multiplexer(MUX) 154 selects between the output of the first PA 155 and the secondPA 150, depending on whether the transmitter is operating in a constantenvelope modulation mode or a variable envelope modulation mode.

The two fixed divide-by-2 blocks 132 and 134 of the PLL 122 allows thesynthesizer to easily generate in-phase (I) and quadrature (Q) carriersignals in four different GSM bands, namely the bands around 850 MHz,900 Mhz, 1800 Mhz, and 1900 MHz, by tuning the VCO 130 appropriatelyaround 3.6 GHz. RF output signals in the DCS/PCS bands are producedusing the first divide-by-2 block 132, while RF output signals in theGSM850/900 band are produced using both divide-by-2 blocks 132 and 134.It should be noted that without the two fixed divide-by-2 blocks 132 and134, two separate VCOs 130 would have to be employed to support the widerange of frequency tuning. In addition, the outputs from both of thefirst divide-by-two block 134 and the second divide-by-two block 132 areinput to a 4×2 MUX 152 that selects the I and Q signals 81 appropriatefor the communications standard being employed.

FIG. 11 is a flowchart illustrating one method 1100 of the presentinvention for operating a dual-use PLL frequency synthesizer. Initially,a determination is made whether the transceiver is currently intransmitting mode or in receiving mode (step 1105). If the transceiveris in receiving mode, the digital processor generates a referencedigital signal at the appropriate frequency for providing an accuratelocal oscillation signal for mixing with the inbound RF signal in thereceiver (step 1110). However, if the transceiver is in transmittingmode, the digital processor modulates outgoing digital data to generatea modulated digital signal (step 1115). Thereafter, the digital signal(reference or modulated) is converted to an analog reference signal(step 1120), and filtered (step 1125) to remove any digital imagespresent in the analog reference signal. The filtered analog signal isup-converted to an output RF signal using a translational loop (step1130).

Thereafter, a determination is made again whether the transceiver iscurrently in transmitting mode or in receiving mode (step 1135). If thetransceiver is in transmitting mode, the output RF signal is a modulatedRF signal provided to the antenna for transmission (step 1140). However,if the transceiver is in receiving mode, the output RF signal is areference RF signal (e.g., local oscillation signal) for mixing with theinbound RF signal in the receiver (step 1145)

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and detailed description. It should beunderstood, however, that the drawings and detailed description theretoare not intended to limit the invention to the particular formdisclosed, but, on the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the claims. As may beseen, the described embodiments may be modified in many different wayswithout departing from the scope or teachings of the invention.

1. A frequency synthesizer for use in a transceiver that is capable ofoperating as a local oscillation generator in a receiving mode and as atransmitter in a transmitting mode, comprising: a digital processoroperable to generate a digital signal, the digital processor capable ofgenerating the digital signal as a modulated digital signal in thetransmitting mode, the digital processor further capable of generatingthe digital signal as a reference digital signal in the receiving mode;a Digital-to-Analog Converter (DAC) operably coupled to receive thedigital signal and operable to convert the digital signal to an analogsignal; a low pass filter operably coupled to receive the analog signaland operable to filter the analog signal to produce a filtered analogsignal; and a phase locked loop operably coupled to receive the filteredanalog signal to up-convert the filtered analog signal to an RF signal,the phase locked loop capable of producing the RF signal as a modulatedRF signal in the transmitting mode, the phase locked loop furthercapable of producing the RF signal as a reference RF signal in thereceiving mode.
 2. The frequency synthesizer of claim 1, wherein thedigital processor is programmed with a reference frequency value toenable the reference digital signal to be generated at a referencefrequency in the receiving mode.
 3. The frequency synthesizer of claim2, wherein the digital processor includes a phase accumulator operablycoupled to receive the reference frequency value and operable togenerate a phase signal corresponding to the reference frequency value,and wherein the digital processor further includes a modulator operablycoupled to receive the phase signal and operable to modulate the phasesignal to produce a complex digital signal.
 4. The frequency synthesizerof claim 3, wherein the modulator is a coordinate rotation digitalcomputer (CORDIC) module.
 5. The frequency synthesizer of claim 3,wherein the digital processor further includes a dual-notch re-quantizeroperably coupled to receive a portion of the complex digital signal andoperable to re-quantize the portion of the complex digital signal toproduce the digital signal.
 6. The frequency synthesizer of claim 6,wherein the digital processor further includes a digital modulatorcoupled to receive outgoing digital data in the transmitting mode, thedigital modulator digitally modulating the outgoing digital data toproduce modulated digital data, and wherein the digital processorfurther includes a summation node connected to receive the referencefrequency value and the modulated digital data in the transmitting mode,wherein the summation node is operable to combine the referencefrequency value with the modulated digital data to produce a combinedsignal, wherein the phase accumulator is operably coupled to receive thecombined signal in the transmitting mode, and wherein the dual notchre-quantizer is operable to produce the modulated digital signal in thetransmitting mode.
 7. The frequency synthesizer of claim 1, wherein thephase locked loop includes a phase and frequency detector connected toreceive the filtered analog signal and a feedback signal and operable toproduce an error signal indicative of a difference in phase or frequencybetween the filtered analog signal and the feedback signal, a chargepump operably coupled to receive the error signal and generate a currentpulse proportional to the error signal, and a loop filter operablycoupled to receive the current pulse and operable to filter the currentpulse to produce a control voltage.
 8. The frequency synthesizer ofclaim 7, wherein the phase locked loop further includes a voltagecontrolled oscillator operably coupled to receive the control voltageand operable to track the phase of the digital signal based on thecontrol voltage to produce the feedback signal as the RF signal, and afrequency divider operably coupled to receive the feedback signal anddivide the feedback signal by an integer divide ratio, and wherein thefeedback signal generated by the voltage controlled oscillator is the RFsignal having a frequency equal to the product of the frequency of thefiltered analog signal and the divide ratio.
 9. The frequencysynthesizer of claim 8, wherein the phase locked loop further includes adivide-by-two frequency divider operably coupled to receive the feedbacksignal from the voltage controlled oscillator and operable to producethe modulated RF signal in the transmitting mode and the reference RFsignal in the receiving mode.
 10. The frequency synthesizer of claim 9,wherein the digital signal includes an envelope signal and a phasesignal, the phase signal being input to the phase locked loop, andwherein the divide-by-two frequency divider includes a firstdivide-by-two frequency divider operably coupled to receive the feedbacksignal from the voltage controlled oscillator and operable to produce afirst RF signal and a second divide-by-two frequency divider operablycoupled to receive the first RF signal and operable to produce a secondRF signal, wherein the reference RF signal is selected from the first RFsignal and the second RF signal in the receiving mode, and wherein thePLL frequency synthesizer further comprises: a first power amplifieroperably coupled to receive the first RF signal and operable to producethe modulated RF signal from the first RF signal and an envelope signalgenerated by the digital processor in the transmitting mode; and asecond power amplifier operably coupled to receive the second RF signaland operable to produce the modulated RF signal from the first RF signaland the envelope signal in the transmitting mode.
 11. A transceiver foruse in a wireless device, comprising: a receiver capable of operating ina receiving mode, wherein the receiver is operable to receive an inboundradio frequency (RF) signal and convert the inbound RF signal to a lowfrequency signal using a reference RF signal; and a frequencysynthesizer capable of operating as a local oscillation generator in thereceiving mode and as a transmitter in a transmitting mode, wherein thePLL frequency synthesizer includes: a digital processor operable togenerate a digital signal, the digital processor capable of generatingthe digital signal as a modulated digital signal in the transmittingmode, the digital processor further capable of generating the digitalsignal as a reference digital signal in the receiving mode, aDigital-to-Analog Converter (DAC) operably coupled to receive thedigital signal and operable to convert the digital signal to an analogsignal, a low pass filter operably coupled to receive the analog signaland operable to filter the analog signal to produce a filtered analogsignal, and a phase locked loop operably coupled to receive the filteredanalog signal to up-convert the filtered analog signal to an RF signal,the phase locked loop capable of producing the RF signal as a modulatedRF signal in the transmitting mode, the phase locked loop furthercapable of producing the RF signal as the reference RF signal in thereceiving mode.
 12. The transceiver of claim 11, wherein the digitalprocessor is programmed with a reference frequency value to enable thereference digital signal to be generated at a reference frequency in thereceiving mode.
 13. The transceiver of claim 12, wherein the digitalprocessor includes a phase accumulator operably coupled to receive thereference frequency value and operable to generate a phase signalcorresponding to the reference frequency value, and wherein the digitalprocessor further includes a modulator operably coupled to receive thephase signal and operable to modulate the phase signal to produce acomplex digital signal.
 14. The transceiver of claim 13, wherein thedigital processor further includes a dual-notch re-quantizer operablycoupled to receive a portion of the complex digital signal and operableto re-quantize the portion of the complex digital signal to produce thereference digital signal.
 15. The transceiver of claim 14, wherein thedigital processor further includes a digital modulator coupled toreceive outgoing digital data in the transmitting mode, the digitalmodulator digitally modulating the outgoing digital data to producemodulated digital data, and wherein the digital processor furtherincludes a summation node connected to receive the reference frequencyvalue and the modulated digital data in the transmitting mode, whereinthe summation node is operable to combine the reference frequency valuewith the modulated digital data to produce a combined signal, whereinthe phase accumulator is operably coupled to receive the combined signalin the transmitting mode, and wherein the dual notch re-quantizer isoperable to produce the modulated digital signal in the transmittingmode.
 16. The transceiver of claim 11, wherein the phase locked loopincludes a phase and frequency detector connected to receive thefiltered analog signal and a feedback signal and operable to produce anerror signal indicative of a difference in phase or frequency betweenthe filtered analog signal and the feedback signal, a charge pumpoperably coupled to receive the error signal and generate a currentpulse proportional to the error signal, a loop filter operably coupledto receive the current pulse and operable to filter the current pulse toproduce a control voltage, and a voltage controlled oscillator operablycoupled to receive the control voltage and operable to track the phaseof the digital signal based on the control voltage to produce thefeedback signal as the RF signal.
 17. A method for synthesizing an RFsignal for use in a transceiver having a transmitting mode and areceiving mode, comprising the steps of: generating a digital signal asa modulated digital signal in the transmitting mode and as a referencedigital signal in the receiving mode; converting the digital signal toan analog reference signal; filtering the analog reference signal toproduce a filtered analog reference signal; and up-converting thefiltered analog reference signal to an RF signal, the RF signal being amodulated RF signal in the transmitting mode and a reference RF signalin the receiving mode.
 18. The method of claim 17, wherein the step ofgenerating the digital signal further comprises: generating a phasesignal corresponding to a reference frequency value; modulating thephase signal to produce a complex digital signal; and re-quantizing aportion of the complex digital signal to produce the digital signal. 19.The method of claim 18, wherein the step of generating the digitalsignal further comprises: receiving outgoing digital data in thetransmitting mode; digitally modulating the outgoing digital data toproduce modulated digital data; combining the reference frequency valuewith the modulated digital data to produce a combined signal, the phasesignal being generated from the combined signal in the transmittingmode.
 20. The method of claim 17, wherein the step of up-converting thefiltered analog signal further comprises: producing an error signalindicative of a difference in phase or frequency between the filteredanalog reference signal and a feedback signal generating a current pulseproportional to the error signal, filtering the current pulse to producea control voltage, and generating the feedback signal to track the phaseof the filtered analog reference signal based on the control voltage.21. The method of claim 17, further comprising the steps of: receivingan inbound radio frequency (RF) signal in the receiver mode; andconverting the inbound RF signal to a low frequency signal using thereference RF signal.